1. Field of the Invention
This invention relates generally to the construction of electronic device assemblies which employ surface mount packages, and more particularly to a method for substantially improving the shock resistance of such electronic assemblies.
2. Description of the Related Art
To build sophisticated electronics, such as computers, stereos, medical instrumentation, and more, discrete electronic components, herein referred to as electronic devices, are mounted onto circuit boards. The circuit boards provide a rigid support for the electronic components, and may additionally provide the interconnections between the electronic devices. As used herein, the term "electronic device assembly" refers to a circuit board having one or more electronic devices mounted on its surface.
Surface mount technology (SMT) is a construction technique for electronic device assemblies in which the terminals of electronic devices are attached to the surface of a printed circuit board (PCB) by solder or some other conductive adhesive. In SMT the device terminals each have a flat (planar) contact surface that rests on corresponding conductive "landing pads" on the PCB surface. SMT may be distinguished from other construction techniques which generally employ "through pin" terminals on their electronic device packages. In these other construction techniques, the device terminals are pins which are placed in holes passing through the circuit board and sealed there by solder or some other conductive adhesive.
SMT fabrication permits components to be mounted to both sides of the PCB. As such, a primary advantage which SMT provides over "through pin" construction techniques is the increased packing density, i.e. the number of components on the PCB per unit of area, which may be achieved by mounting electronic devices on both sides of the PCB. A primary disadvantage of SMT is the decreased reliability relative to the "through pin" construction techniques. In the "through pin" techniques, the terminal physically passes through a hole in the board, thereby providing a strong, shock resistant mechanical coupling to the board. In SMT, the terminals are physically coupled to the board only by conductive adhesive. The terminals, and perhaps even the entire device, may separate from the PCB when the terminal and PCB are subjected to a force that exceed the mounting strength of the adhesive. The mounting strength is often measured by determining the minimum force needed to dislodge a terminal from the landing pad. High reliability and a high packing density are both essential for certain electronic device assemblies such as implantable pacemakers and other implantable medical devices. At least one approach exists for improving the reliability of SMT device assemblies. Referring now to FIG. 1, an electronic device package 102 is shown having leads 104, 106 coupled to a PCB by adhesive joints 108, 110. The mechanical coupling between electronic device package 102 and the PCB is further strengthened by a glob 112 of a non-conductive adhesive.
Electronic device package 102 and leads 104, 106 are illustratively typical of those used for capacitors, resistors, and diodes. These and other two- and three-terminal devices tend to have more mass per terminal than most ICs and other electronic devices with more than three terminals. Consequently, when a PCB is accidentally dropped onto a hard surface or in some other manner subjected to excessive shock forces, the adhesive joints of these devices are subjected to higher stresses than other devices with less mass per terminal. To reduce the likelihood of joint failure, additional adhesive is often applied as one or more globs (such as glob 112 in FIG. 1) between the individual packages and the PCB to provide mechanical reinforcement. This technique requires an additional step during construction of the device assembly for applying the additional adhesive, and also requires that the device packages be spaced apart more than is strictly necessary. The additional space is required to provide the device assemblers with the necessary physical and visual access for applying the non-conductive adhesive globs.
A better method for improving the reliability of electronic device assemblies while increasing the packing density and which does not require additional steps during construction of the device assemblies is desirable.